1. Field of the Invention
The present invention relates to an image pickup device used in a video camera or the like.
2. Description of Prior Art
An image pickup device used in a video camera or the like detects and mixes image signals for different quantities of exposure to provide video signals in a wide dynamic range. The image pickup device has an image sensor which outputs image signals for different quantities of exposure. The image sensor is called, for example, a progressive scan type and it has photoelectric conversion sections or pixels arranged in two dimensions and outputs image signals in scan lines having a number twice that of an ordinary image sensor. In the image sensor, exposure time for pixels in correspondence to a half of the scan lines is changed by an electronic shutter to output image signal Slong for a darker object and image signal Sshort for a brighter object. A mixed signal Smix is obtained from the image signals Slong and Sshort and it is subjected to image processes necessary for a camera such as gamma correction and aperture correction. Further, a mean signal level over a screen is detected, and an iris device is controlled to have a predetermined value according to the mean level of the screen. Such a prior art image pickup device is described in Japanese Patent laid open Publication 322147/1995.
In the prior art image pickup device, as shown in FIG. 1, the image signal Slong saturates at a quantity of incident light L1, and the image signal Sshort saturates at a quantity of incident light L2 which is larger than L1. On the other hand, the dynamic range represented as L2/L1 is constant. Therefore, an image of an object cannot be reproduced well when the dynamic range of the object is larger than the dynamic range L2/L1 of the image pickup device or smaller largely than L2/L1.
An object of the present invention is to provide an image pickup device which can control a dynamic range for a camera in correspondence to the dynamic range of an object.
Another object of the present invention is to provide an image pickup device which performs stable iris control when the dynamic range of the camera is changed.
An image pickup device comprises: an optical lens which transmits a light from an object; an image sensor which detects the light incoming from the optical lens to output image signals for different quantities of exposure; a driver which drives the image sensor; a signal mixer which mixes the image signals for different quantities of exposure outputted by the image sensor to output a mixed signal; a peak level detector which detects a peak level of one of the image signals for different quantities of exposure or a signal derived from the image signals for different quantities of exposure; a peak level judge which judges whether the peak level detected by the peak level detector is within a predetermined range or not; and a controller which controls a ratio of the quantities of light for the image sensor according to the judge result by the peak level judge. Thus, the dynamic range of a camera can be controlled optimally according to the dynamic range of the image sensor by controlling the quantities of exposure for the image sensor according to the peak level which represents brightness of the object.
For example, the peak level detector detects a peak level of the mixed signal outputted by the signal mixer.
For example, the peak level detector detects the peak level of the image signals for a smallest quantity of exposure. Then, the peak level is stabilized while the dynamic range is controlled.
Further, preferably, the image pickup device further comprises: an iris device which limits the light propagating from the optical lens to the image sensor; a signal level judge which receives the mixed signals from the signal mixer and outputs the mixed signals only within a predetermined range; and a mean signal level detector which detects a mean signal level of the image signals received from the signal level judge to output a mean signal level; and an iris driver which controls the iris device according to the mean signal level outputted by the mean signal level detector so as to control the light entering through the optical lens to the image sensor. Thus, the iris device can be controlled stably when the dynamic range of the image pickup device is controlled according to that of the object.
Further, preferably, the image pickup device further comprises: an iris device which limits the light propagating from the optical lens to the image sensor; a mean signal level detector which averages the image signals for the maximum quantity of exposure outputted by the image sensor to output a mean signal level; and an iris driver which controls the iris device according to the mean signal level outputted by the mean signal level detector so as to control the light entering through the optical lens to the image sensor. Thus, the iris device can be controlled stably while controlling the dynamic range of the image pickup device optimally.
Further, preferably, the image pickup device further comprises a limiter which limits a ratio of the quantities of exposure of the image signal controlled by the controller within a predetermined range. As a result, occurrences of the gradation characteristic becoming unnatural and an increase of noises can be advantageously suppressed.
Further, preferably, the image pickup: device further comprises a calculator which calculates a ratio of quantities of exposure of the image signals outputted by the image sensor and a quantity of incident light incoming through the iris device to the image sensor by using calculation with the peak level and the mean signal level. Even if the enlargement of the dynamic range is limited, the iris operation is correlated to the enlargement of the dynamic range. Thus, a ratio of saturated signals is reduced,and an object can be imaged in a suitable state.